This invention relates to apparatus for determining the wet bulb globe temperature (WBGT) index of the surroundings.
As is well known, the wet bulb globe temperature is a practical index of heat stress on living organisms, notably human beings, in hot environments. It is a composite measure of the four factors affecting thermal comfort in man, namely, air temperature, relative humidity, air movement and thermal radiation level. The wet bulb globe temperature is calculated from the formula:
______________________________________ (1) WBGT = 0.7 WB + 0.2 GT + 0.1 DB where WBGT = wet bulb globe temperature index WB = natural wet bulb temperature GT = 6 inch black globe temperature and DB = shaded dry bulb temperature. ______________________________________
The above temperatures may be in .degree. C. or .degree. F. The globe temperature is a composite measure of thermal radiation and convection and conduction exchanges with the environment and is measured with a temperature sensor positioned at the center of the inside of a standard 6 inch diameter metallic globe with a blackened surface.
At present the most common method of measuring the wet bulb globe temperature (hereinafter referred to as the WBGT) of a particular environment is by measuring the three individual temperatures, the wet bulb temperature, the globe temperature and the dry bulb temperature and substituting these measurements into the above formula to determine the WBGT value. Each individual determination of a WBGT with this method requires approximately five minutes. None of the three temperature measurements are made at exactly the same time so that the WBGT value derived does not represent an instantaneous value applicable to the particular environment. Additionally, errors can be made in the individual temperature measurements or in the calculations itself, thus requiring individual checking and verification of these steps. Previously proposed WBGT meters incorporate three sensors, a dry bulb temperature sensor, a wet bulb temperature sensor, and a globe temperature sensor. The globe temperature sensor is usually the standard 6 inch diameter globe which has a time of response to a temperature change in the order of 20 minutes, so that, in environments of rapidly changing heat stress, these instruments do not measure the instantaneous WBGT value.
It is an object of the present invention to a simplified apparatus for determining the wet bulb globe temperature (WBGT) index of the surroundings in which only one temperature sensor is used.
Accordingly, the present invention provides a wet bulb globe temperature measuring instrument for determining the wet bulb globe temperature index of the surrounding comprising a globe member of metallic material provided with a fabric cover jacket over at least a part of its external surface, means for maintaining said cover jacket at least partially wet when exposed to the environment, and a temperature sensing device located within said globe member to provide a measurement of the wet bulb globe temperature index of the surroundings.